The stress of loneliness raises levels of a hormone which helps to trigger tumour growth, scientists believe.
They found that female rats kept in solitude were three times more likely than others to develop cancer.
The tumours the rodents developed were also larger and more deadly.
More than 45,000 women develop breast cancer every year in Britain.
Reducing feelings of loneliness could potentially cut the numbers affected, the team behind the latest study believes.
Their findings show that levels of a stress hormone called corticosterone increased in rats kept in isolation. The researchers believe this hormone affects how cells grow, creating tumours.
The study also found that levels of the chemical stayed higher for longer in isolated rats exposed to stressful situations, such as smelling the odour of fox, than in rodents who lived together.
“This study offers insight into how the social world gets under the skin,” said Gretchen Hermes, from Yale University, who led the study.
Martha McClintock, from the University of Chicago and one of the co-authors of the study, added: “We need to use these findings to identify potential targets for intervention to reduce cancer and its psychological and social risk factors.
‘In order to do that, we need to look at the problem from a variety of perspectives, including examining the sources of stress in neighbourhoods as well as the biological aspects of cancer development.”
The findings were published in the journal Proceedings of the National Academy of Sciences.